PHYS 1444 – Section 003 Lecture #20

PHYS 1444-003, Fall 2011 Dr. Jaehoon Yu

1Thursday, Nov. 10, 2011

PHYS 1444 – Section 003Lecture #20

Thursday, Nov. 10, 2011Dr. Jaehoon Yu

• CH28• Biot-Savart Law• Magnetic Materials• B in Magnetic Materials• Hysteresis

• CH29• Induced EMF and Electromagnetic Induction• Faraday’s Law of Induction• Magnetic Flux• Lenz’s Law



Announcements• Term exam #2

–Date and time: 12:30 – 2:00pm, Tuesday, Nov. 22– Location: SH103–Coverage: Ch. 26 – 3 to what we finish Tuesday,

Nov. 15–A review session on Thursday, Nov. 17, in SH103–Please do NOT miss the exam!!

• Reading assignments– CH28 – 8, 9 and 10



Special Project #6B due to current I in a straight wire. For the field near a long straight wire carrying a current I, show that(a) the Ampere’s law gives the same result as the simple long

straight wire, B=μ0I/2πR. (10 points)

(b) That Biot-Savarat law gives the same result as the simple long straight wire, B=μ0I/2πR. (10 points)

• Must be your OWN work. No credit will be given for for copying straight out of the book or from your friend’s work.

• Due is at the beginning of the exam on Tuesday, Nov. 22.



Biot-Savart Law• Ampere’s law is useful in determining magnetic field

utilizing symmetry• But sometimes it is useful to have another method of using

infinitesimal current segments for B field– Jean Baptiste Biot and Feilx Savart developed a law that a

current I flowing in any path can be considered as many infinitesimal current elements

– The infinitesimal magnetic field dB caused by the infinitesimal length dl that carries current I is

–

• r is the displacement vector from the element dl to the point P• Biot-Savart law is the magnetic equivalent to Coulomb’s law

d

rB =

μ0 I4π

drl× r̂r2

Biot-Savart Law

B field in Biot-Savart law is only that by the current, nothing else.



Example 28 – 11 B due to current I in a straight wire. For the field near a long straight wire carrying a current I, show that the Biot-Savarat law gives the same result as the simple long straight wire, B=μ0I/2πR.

What is the direction of the field B at point P? Going into the page.All dB at point P has the same direction based on right-hand rule.

dy =

Integral becomes

The magnitude of B using Biot-Savart law is

B dB= =Where dy=dl and r2=R2+y2 and since we obtaincoty R =

B =

The same as the simple, long straight wire!! It works!!

02

ˆ

4

dl rIr

μπ

=

0

2

sin4 y

I dyr

μ π

=

2cscR d =2sin

Rd

= 2

Rd

R r

=

2r dR

02

sin4 y

I dyr

μ π

= = 0

0

1 sin4

Id

Rπ

μ

π == 0

0

1 cos4

IR

πμ

π = 0 1

2I

Rμπ



Magnetic Materials - Ferromagnetism• Iron is a material that can turn into a strong magnet

– This kind of material is called ferromagnetic material• In microscopic sense, ferromagnetic materials consist of many tiny

regions called domains– Domains are like little magnets usually smaller than 1mm in length or width

• What do you think the alignment of domains are like when they are not magnetized?– Randomly arranged

• What if they are magnetized?– The size of the domains aligned with the

external magnetic field direction grows while those of the domains not aligned reduce

– This gives magnetization to the material• How do we demagnetize a bar magnet?

– Hit the magnet hard or heat it over the Curie temperature



B in Magnetic Materials• What is the magnetic field inside a solenoid?•

– Magnetic field in a long solenoid is directly proportional to the current.

– This is valid only if air is inside the coil• What do you think will happen to B if we have something

other than the air inside the solenoid?– It will be increased dramatically, when the current flows

• Especially if a ferromagnetic material such as an iron is put inside, the field could increase by several orders of magnitude

• Why?– Since the domains in the iron aligns permanently by the external

field.– The resulting magnetic field is the sum of that due to current and

due to the iron

0B = 0nIμ



B in Magnetic Materials• It is sometimes convenient to write the total field as the

sum of two terms•

– B0 is the field due only to the current in the wire, namely the external field• The field that would be present without a ferromagnetic material

– BM is the additional field due to the ferromagnetic material itself; often BM>>B0

• The total field in this case can be written by replacing μ0 with another proportionality constant μ, the magnetic permeability of the material– μ is a property of a magnetic material– μ is not a constant but varies with the external field

B =

B nIμ=

0B

MB



Iron Core ToroidHysteresis• What is a toroid?

– A solenoid bent into a shape• Toroid can be used for magnetic field measurement

– Why?– Since it does not leak magnetic field outside of itself, it fully contains

all the magnetic field created within it.• Consider an un-magnetized iron core toroid, without any

current flowing in the wire– What do you think will happen if the current slowly increases?– B0 increases linearly with the current.– And B increases also but follows the curved line shown in the graph– As B0 increases, the domains become more aligned until nearly all

are aligned (point b on the graph)• The iron is said to be approaching saturation• Point b is typically at 70% of the max



Hysteresis• What do you think will happen to B if the external field B0 is reduced to

0 by decreasing the current in the coil?– Of course it goes to 0!!– Wrong! Wrong! Wrong! They do not go to 0. Why not?– The domains do not completely return to random alignment state

• Now if the current direction is reversed, the external magnetic field direction is reversed, causing the total field B pass 0, and the direction reverses to the opposite side– If the current is reversed again, the total field B will

increase but never goes through the origin• This kind of curve whose path does not

retrace themselves and does not go through the origin is called the Hysteresis.



Magnetically Soft Material• In a hysteresis cycle, much energy is transformed to

thermal energy. Why?– Due to the microscopic friction between domains as they

change directions to align with the external field• The energy dissipated in the hysteresis cycle is

proportional to the area of the hysteresis loop• Ferromagnetic material with large hysteresis area is

called magnetically hard while the small ones are called soft– Which one do you think are preferred in electromagnets or

transformers?• Soft. Why?• Since the energy loss is small and much easier to switch off the

field • Then how do we demagnetize a ferromagnetic

material?– Keep repeating the Hysteresis loop, reducing the range of B0.



Induced EMF• It has been discovered by Oersted and company in early 19th

century that – Magnetic field can be produced by the electric current– Magnetic field can exert force on the electric charge

• So if you were scientists at that time, what would you wonder?– Yes, you are absolutely right! You would wonder if the magnetic field

can create the electric current.– An American scientist Joseph Henry and an English scientist

Michael Faraday independently found that it was possible• Though, Faraday was given the credit since he published his work before

Henry did– He also did a lot of detailed studies on magnetic induction



Electromagnetic Induction• Faraday used an apparatus below to show that magnetic

field can induce current

• Despite his hope he did not see steady current induced on the other side when the switch is thrown

• But he did see that the needle on the Galvanometer turns strongly when the switch is initially thrown and is opened– When the magnetic field through coil Y changes, a current flows

as if there were a source of emf• Thus he concluded that an induced emf is produced by a

changing magnetic field Electromagnetic Induction



Electromagnetic Induction• Further studies on electromagnetic induction taught

– If a magnet is moved quickly into a coil of wire, a current is induced in the wire.

– If a magnet is removed from the coil, a current is induced in the wire in the opposite direction

– By the same token, the current can also be induced if the magnet stays put but the coil moves toward or away from the magnet

– Current is also induced if the coil rotates.• In other words, it does not matter whether the magnet or

the coil moves. It is the relative motion that counts.



Magnetic Flux• So what do you think is the induced emf proportional to?

– The rate of changes of the magnetic field?• the higher the changes the higher the induction

– Not really, it rather depends on the rate of change of the magnetic flux, ΦB.

– Magnetic flux is defined as (just like the electric flux)–

• is the angle between B and the area vector A whose direction is perpendicular to the face of the loop based on the right-hand rule

– What kind of quantity is the magnetic flux?• Scalar. Unit?• or weber

• If the area of the loop is not simple or B is not uniform, the magnetic flux can be written as

cosB B A BA B AΦ = = = ×

B B dAΦ = ×

2T m× 21 1Wb T m= ×



Faraday’s Law of Induction• In terms of magnetic flux, we can formulate Faraday’s

findings– The emf induced in a circuit is equal to the rate of change

of magnetic flux through the circuit

• If the circuit contains N closely wrapped loops, the total induced emf is the sum of emf induced in each loop

– Why negative?• Has got a lot to do with the direction of induced emf…

Faraday’s Law of InductionBddt

Φ

=

BdN

dt

Φ=



Lenz’s Law• It is experimentally found that

– An induced emf gives rise to a current whose magnetic field opposes the original change in flux This is known as Lenz’s Law

– In other words, an induced emf is always in a direction that opposes the original change in flux that caused it.

– We can use Lenz’s law to explain the following cases in the figures• When the magnet is moving into the coil

– Since the external flux increases, the field inside the coil takes the opposite direction to minimize the change and causes the current to flow clockwise

• When the magnet is moving out– Since the external flux decreases, the field inside the coil

takes the opposite direction to compensate the loss, causing the current to flow counter-clockwise

• Which law is Lenz’s law result of?– Energy conservation. Why?



Induction of EMF

BΦ =

– The angle between the field and the area vector

B dA× =

cosB dA

• How can we induce emf?• Let’s look at the formula for magnetic flux• • What do you see? What are the things that can change

with time to result in change of magnetic flux?– Magnetic field

– The area of the loop



Example 29 – 5 Pulling a coil from a magnetic field. A square coil of wire with side 5.00cm contains 100 loops and is positioned perpendicular to a uniform 0.600-T magnetic field. It is quickly and uniformly pulled from the field (moving perpendicular to B) to a region where B drops abruptly to zero. At t=0, the right edge of the coil is at the edge of the field. It takes 0.100s for the whole coil to reach the field-free

What should be computed first?The flux at t=0 isThe change of flux is

region. Find (a) the rate of change in flux through the coil, (b) the emf and current induced, and (c) how much energy is dissipated in the coil if its resistance is 100Ω. (d) what was the average force required?

The initial flux at t=0.

BΦ =

Thus the rate of change of the flux is

3 30 1.50 10 1.50 10B Wb Wb Φ = =

B

tΦ

=

B A× =

BA = 220.600 5 10T m× = 31.50 10 Wb

0.100s21.50 10 Wb s

31.50 10 Wb =



Example 29 – 5, cnt’d Thus the total emf induced in this period is

Which direction would the induced current flow?The total energy dissipated is

Force for each coil is

E =

=

I =The induced current in this period is

Clockwise

F =

Force for N coil is F =

F =

BdN

dtΦ

= 2100 1.50 10 1.5Wb s V × =

R

= 21.5 1.50 10 15.0100

V A mA= =Ω

Pt = 2I Rt = 22 31.50 10 100 0.100 2.25 10A s J × Ω× =

Il B

NIl B

NIlB = 2 2100 1.50 10 4 5 10 0.600 0.045A T N × × × =

PHYS 1444 – Section 003 Lecture #20

Documents

field b

current i

simple long straight

point pbiotsavart law

pointsthat biotsavarat

thatthe amperes law

infinitesimal current

magnitude of b